Top management is responsible for the success and failure of the entire business performance. The purpose of this research is to examine the relationship between the changes of top managers and firm stock return. A total sample of 1,268 effective start dates and end dates of top managers from 602 firms listed in the Stock Exchange of Thailand (SET) during the years 1999 to 2014 were collected from SETSMART database. Event study methodology was employed to investigate reaction of stock returns around each event. After every effective start date and resignation date was obtained as a center date or event window, daily stock price of 756 trading days (three years) prior to and after each event were retrieved and calculated as daily returns. The stock returns over short and long periods ranging from 3-day, 1-week, 2-week, 3-week, 1-month, 2-month, 3-month, 4-month, 5-month, 6-month, 1-year, 2-year and 3-year returns surrounding the each event window were estimated and further used to computed abnormal return; the difference between the actual and normal return. The results of this study revealed two main findings. Firstly, firms’ stock returns were unfavorable prior to the entering of new manager and significantly improved and turned to positive after two years later. Secondly, on average, following the firms with one-year decline in stock return, the managers were likely to resign and stock return remained unfavorable for years. The findings implied that investors could gain favorable returns from investing in firms associated with new top manager, however, should avoid firms associated with top management resignation.